Fusion reactors are all about getting particles hot enough and keeping them around long enough to react.

The magnetic component of the Lorentz force bends moving charged particles into circular orbits. If the magnetic field is strong enough, you end up with very tight helical orbits where the particle is basically locked onto the field line. The general idea of magnetic confinement is that if you arrange the field lines so they never intersect any structure; to first order, the particles are confined forever. Physically, this is done by taking a solenoid and bending the ends around into a torus so that the field lines 'bite their own tail' and form closed loops. This is essentially what a tokamak like ITER is.

The problem is that the coil turns will always be packed more tightly on the inner-side than the outer-side, you can't escape conservation of current density. This leads to a magnetic field which is not uniform and is stronger closer to the axis. This magnetic gradient leads to a number of second-order drifts in the charged particles' motion which cause them to migrate across field lines and out of the device into the walls.

The way to get around this is to 'twist' the magnetic field lines. Imagine bending a barber pole into a hoop. The idea is that the particles's orbit will now sweep through the inner-side to the top-side to the outer-side to the bottom-side and the drifts in each region will cancel out over some period of time.

Tokamaks do this by driving a large current through the ring of the plasma which creates a magnetic field perpendicular to the one from the solenoid coils and twists the total magnetic field. Basically, you use the conductive ring of plasma as the single-turn secondary of a massive stepdown transformer. The problem is that the primary side of the transformer needs to be driven at a constant dI/dt and must necessarily stop after some period of ramp time. The plasma current needed is also on the order of MA which opens a whole can of instabilities and other difficulties. The next generation like ITER (and some of today's reactors) use other heating methods like neutral beam injection to try to get around this time limit and move into continuous operation mode.

Stellerators, like W7X, use much more complicated coils to produce the helical field directly without needing to drive currents in the plasma. The plasma volume is squashed into a ribbon shape which twists as you move around the closed loop. Since you don't need plasma currents, you are not limited by the transformer ramp life time and can more easily operate indefinitely. No plasma currents also generally means much less instabilities which cool down your plasma. Though they have been around for decades, historically the major downside is the complex 3D geometry was nearly impossible to analyze on paper/PC since there is zero symmetry. Obviously, designing and building those complex 3D coils are much more difficult and expensive also.

Since the instabilities in the tokamak are proving to be more of a problem then assumed in the past, and CAD/simulations are catching up with the 3D design requirements, stellerators are having something of a renaissance lately.

hanelyp wrote:But will stelerators do any better than tokomaks with instabilities? And as a low beta machine a stelerator also needs to be huge to be a fusion generator.

Not really "huge" in respect to ITER, because Stellarators are MHD stable machines at a BETA up to 5% and with a much lower bootstrap current in respect to a symmetrical axis machine like a tokomak.

So far there has been 3 machines proposed to optimize the Wendelstein 7-X into a power producing unit. They are called the HSR5/22, HSR4/18i, HSR3/15, all designed on a 3 GW fusion power basis.The first number represent the "Period" of the machine (add 1 to that number to get the number of times the Plasma twist into the reactor during a complete rotation). The second number indicate the Major Radius.

While the HSR3/15 is the smallest machine (major radius 15 m), it is the HSR4/18i which so far seems the most viable unit from an engineering point of view, thanks to the lower bootstrap energy value and lower Neutron Wall Load. The total "Volume" of the machine should be quite smaller than ITER.I read that there has research going on on a "HSR3/15i" machine to reduce the bootstrap current issue, but I have no detailed info about this.

So a lot of folks in this thread thinks that the tokamakness and stellaratorness are mutually exclusive. This is not true, you can build a hybrid machine to take advantage of all the advantages of tokamaks and stellarators, at the same time.

In fact, it's very useful to build such a hybrid machine, because stellarators shaping field is so stabilizing, it can and will take care of any potential instabilities and even stop disruptions that a tokamak may suffer from. Now... there's the issue that the aspect ratio and size of a hybrid machine may be an issue, because the shaping field may not be able to penetrate into the plasma based on design parameters.

Robthebob wrote:So a lot of folks in this thread thinks that the tokamakness and stellaratorness are mutually exclusive. This is not true, you can build a hybrid machine to take advantage of all the advantages of tokamaks and stellarators, at the same time.

I don't really see how that could be done in engineering terms. Care to elaborate more your idea with some detail?

A 5% Beta does not seem much different than a 3% Beta that I recall has benn quoted for ITER like Tokamaks. It seems a higher Beta (density) would be needed to shrink the machine to economic viability, irregardless of physical viability. Als. This is onl;yo, with such a low Beta (density) the containment time of the ytiple product has to be large, like 100s to 1000s of seconds (?). A 0.1 second containment time for a Polywell may be close to the goal, in a low Beta machine it is only a distant start (admittedly it is a start for the expirimental program that probably says little for expected performance goals).

A perspective that moving charged particles assume a circular orbit about a magnetic field line is often stated in various sources. This is of course a simplification that requires the B field to be homogeneous or even over huge distances compared to the gyroradius of interest. In any B field with a gradient smaller than the size of the universe the orbit will not be round, but oval- elliptical. A circle may be approached but not reached to some (tiny) factor. This simplifies the math some, but I have the impression it also corrupts the understanding. With a steep enough B field gradient, the moving charged particle will never complete one highly elliptical orbit before it is disrupted by some other interaction like collisions, space charge, hitting opposite walls. Appreciation of this is important in considering high Beta conditions where the B field gradient is forced to large proportions. Now the charged particles effectively bounce- rebound off of the B field surface as opposed to becoming enmeshed in B field dominate space. This changes the game considerably. As I have sometimes harped on, it also prioritizes the importance of B field gradients being convex towards the plasma. The Stellarators apparently address this in a time fashion. 1/2 the time the B field gradient (weakening direction) is facing away from the walls. In a Polywell or mostly (?) in a Lockheed design, this is all the time, though recirrculation considerations muddies the picture some. With 1/2 the time the Stellarator is perhaps at a dissadvantage, though with the many knobs it may be controllable, at least much more so than in the Tokamaks.

I was hoping that you could argument your points with more than just a link and without any word on what your actual idea is.

Anyhow, your link refers to a stellarator with added tokamak-style Ohmic Heating field for the sole purpose of determining when MHD instabilities would disrupt the self contained stellarator plasma.

Quoting from the page you linked:

CTH can operate as a pure torsatron, but has an ohmic heating system to drive plasma current allowing our researchers to study disruptions and magnetohydrodynamic (MHD) phenomena in current-carrying stellarator plasmas.

It is not very clear to me as how this would improve a commercial Stellarator fusion machine.

Let's iron out some of the the main differences between a Stellarator and Tokamak for the sake of discussion:

1) The absence of a net plasma current leads to an inherent steady-state operation.2) No need to input power to keep a stable magnetic configuration means that the Plasma can actually burn continuously.3) Highly reduced (up to 20 times) bootstrap current.4) No magnetic Stress on the superconducting coil and reduced/no cyclic stress due to continuous operation.5) No power needed for the Current-drive section of the reactor, meaning no reliability issues and better plant efficiency.

I Really can't think anything that is part of a Tokamak that would improve the already elegant physics and strong stability present inside a Stellarator plasma. But I am always open for suggestions and discussions.

The scope of W7-X is exactly the one to validate (or confute) most of the Physics that went into the design of the commercial HSR4/18i Stellarator I mentioned before.In my opinion, should W7-X hold its expectation, it will move Tokamaks out of the Fusion race.

Maybe you are remembering some older field of research that was trying to obtain smaller Stellarator configurations when Stellarators was not yet optimized (and much bigger than today designs). That field of research has dried out in the last few years as far as I know.

I have no points to argue, because these arent my points, they're 2 decades of research other people have done with regards to Stellarators and Tokamaks, dont look for fights where there isnt one.

But let me say a few things.

1. "The absence of a net plasma current leads to an inherent steady-state operation." This doesnt matter, because neutral injections in Tokamaks have "fixed" the steady-state operation problem, but none of it matters, cus neutral injections can be done in Stellarators, (mainly to bring it to H-mode, something something)

2. "No need to input power to keep a stable magnetic configuration means that the Plasma can actually burn continuously." No man, they need neutral injections to bring the machine to H-mode; even if somehow Stellarators doesnt require H-mode, there's likely to be beta limiting factors, which means you need H-mode, something something.

3. "Highly reduced (up to 20 times) bootstrap current." I'm not very educated with that subject. Speaking of plasma current. The thing is, may it be induced plasma current or flow plasma current (from neutral injections), Tokamaks have strange issues with instabilities and disruptions, which the shaping B field of Stellarators can actually stabilize these issues, which is THE solution to Tokamaks.

5. "No power needed for the Current-drive section of the reactor, meaning no reliability issues and better plant efficiency." I dont know what you mean.

"I Really can't think anything that is part of a Tokamak that would improve the already elegant physics and strong stability present inside a Stellarator plasma. But I am always open for suggestions and discussions."

I dont disagree, I'm saying, it's a Tokamak world out there, and if what needs to be demonstrated is that Stellarators can essentially do anything a Tokamak can do, so people can get on the "right train". Talking about H-mode, neutral injection, plasma current, etc, which are the workhorses and defining features of Tokamaks, but like I said, Stellarators can do all the same sorta things.

There may be design parameters issues we're not aware of that could make Tokamaks have a higher beta (like due to the aspect ratio), from what I understand, a pure Stellarator is shaped like bicycle tires, while Tokamaks are shaped like fat donuts. I think there are factors of beta which depends on aspect ratio. Whether we need a zero plasma current machine (whether this is a constraint we can afford), I dont know. Pure Stellarators are zero induced plasma current machines, and (dont quote me on this) doesnt need neutral injections. But these things are actually good for performance. (H-mode improves performance a good chunk) When the shaping fields of Stellarators actually stabilize plasma current problems, I think we can probably go back to the drawing board to make things be even better... just a thought.

Hybrid machines research are still going strong at Auburn as far as I know. What I was trying to convey is the features of Stellarators and Tokamaks are not mutually exclusive, and it's not conjecture, it's been done, it's still going. The tone of your post sounded like I was just making shit up, but I'm really not... I linked it to prove I wasnt.

Robthebob wrote: The tone of your post sounded like I was just making shit up, but I'm really not... I linked it to prove I wasnt.

No, I was not trying to make it look like if you was making stuff up. I was really interested to know the detailed points about your idea as I know that a hybrid Tokamak/Stellarator had been researched before but (for my knowledge) it didn't bring out any meaningful results, so I was really interested to know if you had more information.

Sorry if I gave you a wrong impression, it was not what I meant. Unfortunately sometimes is hard to fully transfer one's intentions trough few words of a post. That's one of the few reasons why I hate chatting in internet instead than in real life.

Let me check your points tonight when I am back from work and reply to you. Always for the sake of knowledge sharing and discussion.

Giorgio, I can vouch for Rob, he has a clue. He has done the work in his undergrad, and is looking to do more.

The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)

"There may be design parameters issues we're not aware of that could make Tokamaks have a higher beta (like due to the aspect ratio), from what I understand, a pure Stellarator is shaped like bicycle tires, while Tokamaks are shaped like fat donuts. I think there are factors of beta which depends on aspect ratio. Whether we need a zero plasma current machine (whether this is a constraint we can afford), I dont know. Pure Stellarators are zero induced plasma current machines, and (dont quote me on this) doesnt need neutral injections. But these things are actually good for performance. (H-mode improves performance a good chunk) When the shaping fields of Stellarators actually stabilize plasma current problems, I think we can probably go back to the drawing board to make things be even better... just a thought."

My indoctrination one day from Paul Koloc suggests that you want to go the opposite direction. Bicycle tires would be worse than car tires. What you want is over-inflated ATV tires. What you really want is a spheromak.

But what I really want is beta approaching 1. Or a system that works so well in short pulses at small scale that beta doesn't matter, and confinement is inertial. Or a really workable electrostatic system.